The problem was spare had to be good at fighting case heir kicked the bucket. Then you had to figure out how to keep spare from killing heir. Standard techniq

The problem was spare had to be good at fighting case heir kicked the bucket. Then you had to figure out how to keep spare from killing heir. Standard technique was to encourage them to go conquer someplace else. This was also an important driver of the crusades, which had the bonus of sending that heavily armed relative far away.